'unmanaged class' : a managed class cannot derive from an unmanaged class
You attempted to use an unmanaged class as a base class for a managed class
The following sample generates C3253:
#using <mscorlib.dll> struct S { // unmanaged class }; /* use the code below to resolve the error __gc struct S { }; */ __gc class C : public S { // C3253 }; void main() { }